Hypodermic syringes are widely used in the medical arts for administering medicaments and for drawing body fluid samples. Generally, hypodermic syringes include a fixedly or removably attached metal needle that has a sharpened distal point for penetrating vial stoppers or a patient's skin. Hypodermic syringes and needles have been used for many years with few problems reported, taking into consideration the vast numbers and needles used. More recently, with the recognition of viral diseases that are transmitted by body fluids and greater sensitivity of the need to protect health care workers from inadvertent contact with previously used needles (commonly referred to as “sharps”) as well as the need to reduce misuse of improperly disposed of needles and syringes, syringes and needles that include provisions to prevent reuse have been developed.
Provisions intended to protect health care workers from accidental needle sticks and prevent reuse of needles and syringes include a variety of sharps collector systems that are widely used in health care facilities. Other developments include needle attachments that may be readily broken off by practitioners once the syringe has completed its intended use. A variety of shielding mechanisms have been developed which are intended to shield the needle or sharp after it has been used, thus reducing the risk of an accidental needle stick. While many of these developments have reduced the incidence of inadvertent exposure of healthcare workers to sharps, most of these devices can readily be overcome by an individual determined to obtain and misuse a hypodermic syringe and needle. As a result of this problem, further developments in the art of hypodermic syringes have resulted in syringes with needles that withdraw into the body of the syringe once their intended use is completed. These are often referred to as retracting needle syringes.
Current conventional (i.e., non-retracting needle) syringes are considered by users to be virtually fault-free and reliable. They are used for a variety of different procedures involving both “one-shot” fill and inject procedures, as well as more complex mixing measuring and delivery functions. For retractable syringes to replace these functional, utilitarian and reliable conventional syringes, retractable syringes should not significantly interfere with the users current practices and they should be substantially reliable. In addition, in view of the fact that current conventional syringes are often manufactured at rates of several hundred per minute and their cost is generally not a significant factor in their usage, retractable syringes must be cost-effective to manufacture.
Most of the available retracting needle devices are somewhat complex, and many require manufacture and assembly of parts with potentially difficult assembly or tight tolerance requirements. Many of the designs depend upon a careful application of forces by the practitioner to draw and expel fluids from the syringe. Also, if the tolerances between the multiple components of the device are not carefully adhered to during manufacture and assembly, normal usage may result in premature activation of the retraction function of the syringe. The problem of premature activation of the retraction function is a problem with many available retracting needle syringes, particularly those that rely upon application of compressive force on the syringe stopper to activate the retraction mechanism. Many of the available retracting needle syringes have substantial undeliverable “dead volumes” that confound the practitioners need for accurate delivery of medicaments from the syringe or that may waste a substantial percentage of a high cost medicament that is left in the dead volume space. The problem of dead volumes may be associated with a syringe that relies on displacement of the plunger rod with respect to the syringe barrel. Previous syringe designs rely on either force against the stopper or displacement of the plunger rod to cause activation of the retraction mechanism.
Accordingly, a need exists for a selectively retractable syringe that can withstand normal forces during injection and avoid premature activation of the retraction mechanism. Moreover, there is a need to reduce the volume of waste space in the syringe and prevent leakage of medication from the syringe.